There is a waveguide slot array antenna in which a plurality of slots are formed in a waveguide, and electromagnetic waves are radiated from these slots.
When exciting the slots as radiating elements by propagating waves in this waveguide slot array antenna, it is necessary to suppress reflection from each radiating element. To suppress reflection from each slot, it is possible to arrange a post near the slot and cancel reflection from the slot by reflection from the post, or arrange a pair of two slots as one radiating element and cancel reflections from the two slots.